In July, Homeland Security, the Secret Service, and the National Cybersecurity and Communications Integration Center warned companies to check their cash register systems for a malware dubbed "Backoff."

Some big-name businesses, like Supervalu and UPS, have already stepped forward to say they found the malware, but the Secret Service now estimates that more than 1,000 other businesses have been affected, too.

Criminals can use the Backoff malware to steal unencrypted data from cash register systems they gain access to. The credit card information of millions of Americans is reportedly being sold on the black market, because hundreds of companies don't realize their systems have been affected by the malware.